The present invention relates to a method for controlling at least one hydraulic cylinder in a work machine, which hydraulic cylinder is arranged to move an implement in relation to a part of a vehicle, with the hydraulic cylinder being controlled by a hydraulic machine.
The invention will be described below in connection with a work machine in the form of a wheel loader. This is a preferred but in no way limiting application of the invention. The invention can also be used for other types of work machines (or work vehicles), such as an excavator loader (backhoe) and excavating machine.
The invention relates, for example, to controlling lifting and/or tilting cylinders for operating an implement.
More precisely, the invention relates to a control system which comprises a hydraulic machine which functions as both pump and motor. The hydraulic machine is connected in a driving manner to an electric machine which functions as both motor and generator.
The hydraulic machine therefore functions as a pump in a first operating state and supplies pressurized hydraulic fluid to the hydraulic cylinder. The hydraulic machine also functions as a hydraulic motor in a second operating state and is driven by a hydraulic fluid flow from the hydraulic cylinder. The electric machine therefore functions as an electric motor in the first operating state and as a generator in the second operating state.
The first operating state corresponds to a work operation, such as lifting or tilting, being carried out with the hydraulic cylinder. Hydraulic fluid is therefore directed to the hydraulic cylinder for movement of the piston of the cylinder. On the other hand, the second operating state is an energy recovery state.
It is desirable to achieve a method for controlling a hydraulic cylinder, preferably for a lift function and/or tilt function, that provides smooth operation and means that the driver is subjected to fewer shocks and jerks.
According to an aspect of the present invention, a method is provided comprising the steps of detecting initiation of a movement of the implement that is such that the piston in the hydraulic cylinder is moved in a first direction, of driving the hydraulic machine in a first rotational direction, prior to the movement of the implement taking place, so that a line from the hydraulic machine is pressurized, which line is arranged to connect the hydraulic machine to the side of the cylinder toward which the piston will be moved during the movement of the implement.
The fact that the movement of the implement has been initiated is preferably detected directly via an input from an operator of the vehicle, such as a movement of a lifting lever.
The method is primarily applicable for a lowering movement of a load to avoid shocks, but can also be utilized for a lifting movement of the load arm on the work machine, or alternatively for a tilting movement of the implement.
Further preferred embodiments and advantages of the invention emerge from the following description.